The present invention relates to a drive device for a thermal recording head of a type used in a printer or facsimile device. More particularly, the invention relates to a thermal recording head drive device which is capable of performing half-tone recording.
FIG. 1 is a side view outlining the general arrangement of a thermal recording head device. In FIG. 1, reference numeral 1 designates a thermal recording head, 2 a recording sheet, 3 a platen roll, and 4 drive rolls. A conventional thermal recording head device of this general type is composed of (1) a linear-type head including heat generating resistance elements arranged in an array extending in a direction perpendicular to the surface of the drawing having a length at least equal to the width of the heat-sensitive sheet 2, (2) the heat-sensitive sheet 2, which is driven past the head 1 by the drive rolls 4, and (3) the platen roll 3 for pressing the heat-sensitive sheet 2 against the head 1.
In a printer or facsimile device utilizing a conventional thermal recording system as described above, half-tone recording is carried out using an "artificial" method in which one recording element (dot) is divided into a plurality of areas. In recording data with a high density, a large number of areas are recorded. In order to perform recording with high fidelity, the number of areas obtained by so dividing one recording element must be large. However, the minimum size of an element in a recording head is limited. That is, it is impossible to reduce the size of the element beyond certain limits. On the other hand, if it is required to perform recording with a large number of gradations, the area of each recording element must be relatively large, with the result that the recording is low in resolution.
Another half-tone recording method is known in the art in which, based on the fact that the recording density vs. voltage application time characteristic of a heat-sensitive sheet is linear to some extent, in recording at a low density the voltage application time is decreased, while in recording at a high density the voltage application time is increased. This method is free from the above-described drawback of low resolution because one recorded element (dot) corresponds to one element of the head. However, the method is still disadvantageous in that, for high speed recording with a linear-type head having a length equal to the width of a recording sheet, it is necessary to provide pulse-width modulator circuits in a number equal to the total number of recording elements on the head. For instance, in the case of a head having a length equal to the short side (257 mm) of a size "B4" sheet and a density of a 8 dots/mm, the number of elements is 2056 (257.times.8). That is, it is necessary to provide 2056 pulse-width modulator circuits. This is not practical.
Further, it has been a problem in prior art thermal recording head drive devices that the recording density varies considerably as the temperature of the heat-sensitive recording sheet or the substrate temperature of the recording head changes. If these temperatures change outside a rather narrow operating range, the recording density tends to become irregular and the recorded image low in quality.
Accordingly, it is an object of the present invention to provide a thermal recording head drive device of low cost and small size in which heat-sensitive recording operations of half-tone recording can be carried out at a high speed and with a high resolution.
It is a further object of the present invention to provide such a thermal recording head drive device in which a uniform recording density is achieved throughout a broad temperature range.